It is known to support a pair of balancer shafts with bearing bores provided in parts of journal walls located on either side of the cylinders and to rotate them in synchronism with the crankshaft for the purpose of cancelling the secondary unbalanced inertia force which is produced in most commonly used reciprocating piston engines for vehicles or four-stroke, in-line, four-cylinder engines. Supply of lubricating oil to the bearings of balancer shafts and a crankshaft is generally carried out by using lubricating oil passages branching out from a main gallery extending in a cylinder block side wall in parallel with the cylinder row and into different journal walls as disclosed in Japanese utility model laid open publication No. 62-156622 and Japanese utility model laid open publication No. 63-4318.
Since lubricating oil supply passages in journal walls are typically formed either by machine drilling or by combination of casting and machine drilling, the manufacturing process becomes increasingly complex, and the journal walls tend to have a reduced rigidity as the number of the parts to be lubricated in each journal wall increases, and the lubricating oil supply passages crisscrosses the interior of each journal wall. Further, unevenness of the distribution of lubricating oil to different bearings could become a problem.